tests: Use get_bootloader_message_blk_device() to find /misc.

Since commit fb00d82f has added
get_bootloader_message_blk_device() as an API, switch the tests-local
implementation to it.

Test: recovery_component_test on angler.
Test: recovery_component_test on a local build that doesn't have /misc.
Change-Id: I4f5f542cb9ef58292c587a677da73d8822db7262